1. Field of the Invention
This invention relates to an optical distance measuring apparatus, and more particularly to a distance measuring apparatus best suited for the optical focus detecting apparatus of a camera.
2. Description of the Prior Art
A patent relating to a camera focus detecting method (distance measuring method) is disclosed in Japanese Laid-open patent application No. 83524/1976 (this Japanese Patent application claims priority on the basis of U.S. application Ser. No. 529,573 and corresponding British applications were patented as British Pat. Nos. 1,521,100; 1,521,268 and 1,521,269). The distance measuring method corresponds to the distance measuring principle of the range finder, and uses a pair of photoarrays instead of the eye to scan the optical images thereon and thereby operate the correlation of the outputs of the photoarrays, thus determining the position of an object to be photographed from the position of the optical system when the correlation is best.
The principle of the optical system of this conventional focusing apparatus will hereinafter be described.
Referring to FIG. 1 of the accompanying drawings, a phototaking lens 1 has its focus adjusted so that the image of an object to be photographed is formed on a predetermined imaging plane 2 such as a film surface. A pair of imaging lenses 3 and 4 for distance measurement have an equal focal length f and both of them are fixed in the directions of the optical axes thereof and are disposed with a distance L between the optical axes thereof. One-dimensional photoelectric element arrays 5 and 6 are disposed rearwardly of the distance measuring lenses 3 and 4, respectively. These arrays 5 and 6 are identical in arrangement direction of their photoelectric elements and both of them are oriented perpendicular to the optical axes. One distance measuring lens 3 is movable in the arrangement direction of the photoelectric elements and is operatively associated with the phototaking lens 1 as shown by a dotted line so that the phototaking lens 1 is moved in the direction of its optical axis in response to movement of the distance measuring lens 3.
Assuming that the distance between the distance measuring lenses 3, 4 and the corresponding arrays 5, 6 is determined so as to be equal to the focal length f of the lenses 3, 4, when the object lies at infinity, clear images of the object by the distance measuring lenses 3 and 4 are formed at the same positions on the corresponding arrays 5 and 6, namely, such that the relative positional relations between the object images and the arrays 5, 6 are identical. At this time, the phototaking lens 1 forms the object image clearly on the predetermined imaging plane 2. On the other hand, when an object 7 lies at a finite distance a as shown in FIG. 2 of the accompanying drawings, the distance measuring lens 3 is displaced by a distance x in the arrangement direction of the photoelectric elements of the array 5, whereby the relative position of the array 5 and the object image thereon can be matched to the relative position of the array 6 and the object image thereon. At this time, the phototaking lens 1 is moved by a predetermined amount toward the object in response to the displacement x of the distance measuring lens 3. The amount of displacement x of the distance measuring lens 3 effected so that the said two relative positions are matched to each other corresponds to the object distance a and therefore, focusing operation can be accomplished by moving the phototaking lens 1 in accordance with the amount of displacement x.
An in-focus detecting apparatus is also known in which a mirror is swung to thereby displace the optical image as shown in FIG. 3 of the accompanying drawings. This distance measuring system is comprised of a fixed mirror 8, a rotatable mirror 9, a reflecting member 10, a pair of imaging lenses 11 and 12 for distance measurement, and a pair of photoelectric element arrays 5 and 6. The rotatable mirror 9 is rotated so that the relative positional relations between the optical images by the distance measuring lenses 11, 12 and the arrays 5, 6 are matched to each other. This matching is detected by the photoelectric outputs of the arrays 5 and 6. The phototaking lens 1 is moved in the direction of its optical axis in response to the rotatable mirror 9, whereby it is focus-adjusted.
However, as is apparent from FIGS. 1 and 2, for an object lying at infinity, clear images are formed on the arrays 5 and 6, while for an object lying at a finite distance, particularly, at a very short distance, the images on the arrays 5 and 6 become greatly blurred (unclear). Accordingly, detecting the matching between said two relative positions with respect to such blurred images results in reduced accuracy and, in some cases, it is electrically very difficult.